Preliminary Performance Assessment for Deep Borehole Disposal of High-Level Radioactive Waste
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Preliminary Performance Assessment for Deep Borehole Disposal of High-Level Radioactive Waste Peter N. Swift1, Bill W. Arnold1, Patrick V. Brady1, Geoff Freeze1, Teklu Hadgu1, and Joon H. Lee1 1 Sandia National Laboratories, Albuquerque, New Mexico, USA 87185 ABSTRACT Deep boreholes have been proposed for many decades as an option for permanent disposal of high-level radioactive waste and spent nuclear fuel. Disposal concepts are straightforward, and generally call for drilling boreholes to a depth of four to five kilometers (or more) into crystalline basement rocks. Waste is placed in the lower portion of the hole, and the upper several kilometers of the hole are sealed to provide effective isolation from the biosphere. The potential for excellent long-term performance has been recognized in many previous studies. This paper reports updated results of what is believed to be the first quantitative analysis of releases from a hypothetical disposal borehole repository using the same performance assessment methodology applied to mined geologic repositories for high-level radioactive waste. Analyses begin with a preliminary consideration of a comprehensive list of potentially relevant features, events, and processes (FEPs) and the identification of those FEPs that appear to be most likely to affect longterm performance in deep boreholes. The release pathway selected for preliminary performance assessment modeling is thermally-driven flow and radionuclide transport upwards from the emplacement zone through the borehole seals or the surrounding annulus of disturbed rock. Estimated radionuclide releases from deep borehole disposal of spent nuclear fuel, and the annual radiation doses to hypothetical future humans associated with those releases, are extremely small, indicating that deep boreholes may be a viable alternative to mined repositories for disposal of both high-level radioactive waste and spent nuclear fuel. INTRODUCTION The concept of using deep borehole repositories for permanent isolation of radioactive materials has, because of its simplicity, modularity, ease of construction, relatively low cost, and safety, been proposed and investigated intermittently for decades (see Refs. 1 through 13). The earliest proposals for deep borehole disposal considered direct disposal of liquid high-level wastes from reprocessing (e.g., Ref. 1); subsequent analyses have considered disposal of solid wastes of various types, including glass high-level waste, spent nuclear fuel, and surplus weapons-grade plutonium. Although published analyses to date have concluded that the overall concept has the potential to offer excellent isolation, disposal programs worldwide have focused on mined repositories, in part because of the availability of proven mining technologies at the time that national policy decisions were made, and in part because of concerns about the feasibility of retrieving waste from deep boreholes. Advances in drilling technologies over the last several decades (Ref. 14) suggest that the construction of deep boreholes
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